CLASSIFICATION OF
PHYLUM-CHORDATA
The classification of phylum—Chordata
is divided into three subphyla, i.e.,
Urochordata(Tunicata),
Cephalochordata and Vertebrata.
Subphyla—Urochordata and
Cephalochordata are often referred to
as protochordates and are exclusively
marine.
Details of each subphyla and its
numerous classes are given below
SUBPHYLUM-UROCHORDATA
The body is covered by a testa (tunic) composed largely of
tunicine. So, they are commonly called as tunicates. The
notochord is present only in the tail of the larva and
disappears in the adult. Coelom is absent, digestive tract is
complete, respiratory system has two to numerous gill slits in
the pharyngeal wall. Circulatory system is open and has
simple tubular heart. Larva undergoes retrogressive
metamorphosis, e.g., Ascidia, Herdmania (sea squirt), Salpa,
Doliolum.
SUBPHYLUM- CEPHALOCHORDATA
In cephalochordates, the notochord extends the entire length
of the body and passes a head of the nerve cord in front’
Notochord persists throughout the life. A true enterocoelous
coelom is present. It is, however reduced in the pharyngeal
region of atrium, e.g., Branchiostoma (amphioxus).
SUBPHYLUM-VERTEBRATA
The subphylum—Vertebrata represents advanced or higher
chordates which include vertebrates or craniates. Notochord
is present only in the embryonic stage. In adults, it is replaced
by a vertebral column consisting of ring-like bones called
vertebrae.
The anterior part of the notochord is however, replaced by
skull or cranium to protect brain, hence named Craniata
Pharyngeal gill slits persist only in adults of lower forms.
Circulatory system is closed type with ventrally situated heart
and hepatic portal system. Respiratory pigment haemoglobin
of  is present in blood corpuscles. Excretory organs consist
of a pair of kidneys. They are unisexual and lack
reproduction.